Large-scale vivid metasurface color printing using advanced 12-in. immersion photolithography

Nanostructures exhibiting optical resonances (so-called nanoantennas) have strong potential for applications in color printing and filtering with sub-wavelength resolution. While small scale demonstrations of these systems are interesting as a proof-of-concept, their large scale and volume fabrication requires deeper analysis and further development for industrial adoption. Here, we evaluate the color quality produced by large size nanoantenna arrays fabricated on a 12-in. wafer using deep UV immersion photolithography and dry etching processes. The color reproduction and quality are analyzed in context of the CIE color diagram, showing that a vivid and vibrant color palette, almost fully covering the sRGB color space, can be obtained with this mass-manufacturing-ready fabrication process. The obtained results, thus, provide a solid foundation for the potential industrial adoption of this emerging technology and expose the limits and challenges of the process.


High brightness color palette image
The optical microscope image of the color palette in the main text (Figure 2b) was taken with brightness level is set to maximum possible, before the background color starts to deviate from black. While it provides a general overview of color quality, some areas with low reflection remain dark and it is difficult to estimate the color. Figure S1 is a color palette image taken at higher light source brightness (same magnification x10 and NA = 0.2), revealing the colors in the left bottom corner. Although colors become brighter, Si 3 N 4 layer reflection becomes observable: background deviates from target black and influences the color perception. Figure S1. Optical microscope image of color palette taken at high light source brightness

Oblique incidence and large NA objectives
We have performed optical microscope imaging of colour palette to trace the color changes as a function of the objective numerical aperture (NA). Results are shown in Figure S2. NA = 0.13 (panel a) and NA = 0.2 (panel b) color palettes were taken as a one shot image, while NA = 0.4 (panel c) was stitched from two images to include all colors. Besides obvious difference in the resolution and sharpness of the image, colors do not significantly deviate, nonetheless, become slightly less vivid and saturated for higher NA.
Higher numerical aperture focuses/collects more angles of incidence, therefore we looked into behaviour of resonances at oblique incidence in order to explain the difference. Figure S3 shows numerical simulations of reflection spectra at various angles of incidence (β ) for p-polarized light (electric field vector parallel to the plane of incidence). Angles of incidence β from Figure S2. Optical microscope images of color palette taken with objective of different NA: (a) 0.13, magnification x5; (b) 0.2, magnification x10; (c) 0.4, magnification x20.
2/5 0 • to 30 • correspond to NA of the objectives used in this work (NA = 0.13 to 0.5). Panels (a) to (d) in Figure S3 correspond to reflection spectra of the selected designs in letters "N", "S", "L", "M" (Figure 3 of main text). Figure S3. Numerical simulations of reflection spectra as a function of p-polarized light incident angle for "N", "S", "L", "M" nanostructure designs (a,b,c,d correspondingly). Insets show structure schematics. Figure S3 provides evidence of noticeable red shift relative to incidence angle for larger Si disks ("S","L","M" designs), with an exception of "N" letter design where behaviour is more complex. Other polarization (s-polarization, electric field vector perpendicular to the plane of incidence) does not exhibit same behaviour as we increase the angle of incidence: spectra stays almost unchanged with negligible blue shift, therefore, we do not show it here. In the higher numerical aperture objective light is incident at all angles and the obtained reflection spectra is the result of the sum of those angles. Red shift of p-polarization as a result of oblique incidence can be one of the reasons of the experimental results red shift relative to numerical simulations spectra in Figure 3 in the main text. Numerical simulations there were estimated only for normal incidence. S-polarization oblique incidence may only contribute to slight broadening of the spectra. Figure S4 demonstrates the fabrication flow (panel a) combined with the SEM images of nanostructure shapes evolution (panel b) after the following the fabrication steps: photolithography mask, etch of SoC mask and Si, etch mask removal (final structure), all fabrication details are given in Methods. SEM images are given from top and side/ 30 • angle view relative to the sample surface. For evaluation we selected "M" letter design (D = 170 nm and G = 120 nm). SEM images of photoresist mask expose small irregularities in the target circular mask shape, disk sizes were taken as an average of two orthogonal measurements. After SoC and Si etch a straight sidewall is observed, showing the quality of photoresist pattern transfer into Si nanostructures. Final image after SoC removal exposes a slight tapering from the Si etch estimated to be 4.6 • , calculated from average bases and height of the truncated cone. Slight deviations from circular shape and tapering contributed into overall experimental spectra broadening relative to numerical simulations ( Figure 3 of the main text).

Nanostructure shape analysis and tapering
Numerical simulations of nanostructure tapering effect on reflection spectrum are shown in Figure S5. Simulations were performed at normal incidence with tapering angle α up to 10 • , almost double the estimated angle from experimental SEM images. In simulations the median diameter and height were fixed corresponding to design, varying angle determining the values of bases. Results demonstrate slight spectral broadening and blue shift of the tapered nanostructure resonances relative to cylinder shape. Therefore, we can conclude tapering and deviations from circular shape contributed into overall experimental spectra broadening relative to numerical simulations ( Figure 3 of the main text). Figure S4. (a) Fabrication process flow schematics; (b) SEM images of nanostrusture shape after different fabrication steps: after photolithography development, after SoC and Si etch, after mask removal (final structure); scale is 100 nm. Figure S5. Numerical simulations of reflection spectra as a function of tapering angle (α) for "N", "S", "L", "M" nanostructure designs (a,b,c,d correspondingly). Arrows show the plot shift direction as α increases. Insets show structure schematics.